The invention concerns a process for production of zeolites with a high Si/Al atomic ratio, especially zeolites from the pentasil group.
Zeolites belong to the class of aluminosilicates and are characterized by a defined pore volume. The structure is made up of [SiO4]xe2x88x924xe2x88x92 and [AlO4]5xe2x88x92 tetrahedra. The Si/Al atomic ratio can be varied over a wide range. A negative excess charge is produced by incorporation of Al3+ in the lattice, which is compensated for by cations (mostly alkali or alkaline earth metal cations). After exchange of the cations with the ammonium ions and subsequent calcining the so-called H form of zeolite is obtained. This represents a strong solid acid and has a high ion exchange capacity (IEC). Aluminum or silicon can also be partially replaced in a zeolite by other atoms like boron, iron or gallium so that a broad spectrum of materials is accessible.
As porous solids with defined pore radii, adjustable acidity and hydrophobicity, high heat stability and ion exchange capacity, zeolites have found numerous technical applications, for example, in petrochemistry, where zeolites of the pentasil type, for example, are used in H form as tailor-made catalysts in cracking processes. Zeolites also often find application as drying agents.
In addition to the advantage of easy removability and recovery of the catalyst, the use of zeolites in heterogeneous catalysis generally offers shape-selective catalysis, in which the formation of byproducts is strongly suppressed.
Zeolites have thus far generally been produced under xe2x80x9chydrothermalxe2x80x9d conditions, in which a silicon source, an aluminum source, optionally an organic template and a mineralizer (for example, alkali metal hydroxides or fluorides or HF) were converted at more than 100xc2x0 C. under pressure in the pH range between 4 and 14. Hydrothermal syntheses of zeolites with an Si/Al atomic ratio of more than 20, for example, pentasil, are run in autoclaves at temperatures that are generally higher than 130xc2x0 C., for example, at 180xc2x0 C. Low-temperature syntheses of zeolites below the boiling point of water have thus far only been known for aluminum-rich phases, like zeolite X.
The development of a low temperature synthesis of zeolites with a high Si/Al atomic ratio is of technical interest, because the use of autoclaves can be dispensed with on this account. In addition to safer handling, by reducing the reaction temperature to the production costs are reduced. Additionally advantages of pressureless low temperature synthesis are: fewer sources of hazard, simpler operation, simplification of sampling during synthesis, possibilities of on-line investigation during synthesis, better control possibilities during synthesis and more precise control of the reaction temperature by elimination of the pressure parameter.
The underlying task of the invention is to provide a process for production of zeolites with a high Si/Al atomic ratio (high silica) of 40 to ∞, especially zeolites from the pentasil group, which offers the advantages just described over the xe2x80x9chydrothermalxe2x80x9d process. Pentasils are understood according to the invention to also include silicalites that contain no aluminum.
The process according to the invention is run by conversion of amorphous polysilica (polysilacic acid) with a source of aluminum ions and optionally a template at temperatures of up to about 100xc2x0 C. and is characterized in that ammonium hydroxide and/or an alkali metal silicate is added to the reaction solution. A pH value in the range from about 11 to 13, preferably 11 to 12, is generally preferred.
Sodium water glass is preferably used as alkali metal silicate, since this is cheaper than other alkali metal silicates.
The conversion is preferably run at temperatures in the range from 40 to 90xc2x0 C., especially from 40 to 80xc2x0 C., in which the reaction rate being reduced at lower temperatures.
Silica hydrogels or pyrogenic silicas are preferably used as amorphous polysilica. Precipitation silicas, however, can also be used.
Essentially all templates used in known zeolite syntheses are appropriate as template. Tetrapropylammonium compounds, for example, tetrapropylammonium bromide, are preferably used.
An Si/Al atomic ratio of more than 75, especially more than 150, is preferably chosen.
Seed crystals of the reaction products can be added to the reaction solution, which is particularly advantageous in synthesis in the presence of ammonium hydroxide.
The reaction solution can also be concentrated in the presence of a support (for example, a vitreous, ceramic or metallic material), in which the reaction product is deposited on the support and enters into a strong bond with it (drying synthesis). Such composite materials can be used as supported catalysts.
Zeolites with a high Si/Al atomic ratio of 40 to ∞ are also an object of the invention, especially zeolites from the pentasil group, which can be obtained according to one of the methods just described with (a) an average particle size of about 0.3 to 0.6 xcexcm, in which 90 wt % of the particles lie in the range from about 0.1 to 0.7 xcexcm, and (b) a crystallinity of more than 95%.
These parameters can be determined as follows:
The average particle size and particle size distribution, as well as the morphology of the crystallites are determined by scanning electron microscopy (SEM), since the morphology of the crystallites with a particle size of less than 1 xcexcm can no longer be determined using a light microscope.
The crystallinity is primarily determined by x-ray powder diffractometry. The intensity of the measured reflections is determined by a comparison with the intensity of a fully crystalline standard. For this purpose the integral intensity of a selected reflection Ihkl is used relative to the same reflection of the 100% crystalline standard:
X-ray crystallinity=[Ihki (sample)/Ihkl (standard)]X 100%
The products according to the invention have a crystallinity of more than 95%.
According to the first variant of the process according to the invention (ammonium-supported synthesis) ammonium hydroxide is added to the reaction solution. This synthesis is characterized by products with a very uniform crystallite size distribution and is explained in example 1.